Ballast water refers to sea water which is charged into a ballast tank of a ship to maintain balance of the ship when the ship sails without cargo.
As marine transportation rates have gradually increased along with an increase in international trade, the number of ships used has increased, and the ships used are becoming larger. As a result, the amount of ballast water used in ships has greatly increased. As the amount of ballast water used in ships is increased, occurrence of damage to indigenous ocean ecosystems attributable to foreign marine creature species is also increased. To solve such international environmental issues, in 2004, IMO (international maritime organization) established ‘International convention for the control and management for ship's ballast water and sediments’. Since 2009, ballast water treatment devices have been obligatorily installed in new constructed ships.
As examples of conventional methods for treating ballast water, a method of exchanging ballast water on the sea and a method of treating ballast water on the ground have been used. However, these conventional methods have the disadvantage of being inefficient. Therefore, recently, a method using a ballast water treatment device installed in a ship is widely used. Particularly, a filtration method using a filter is mainly used as the ballast water treatment device. As ships are becoming larger, a need for treatment of a large amount of ballast water is increased. Accordingly, multi-cage type ballast water filtering devices which can treat a large amount of ballast water are recently used.
The term ‘multi-cage type ballast water filtering device’ refers to a device in which a plurality of filtering units, each of which includes a filter and a device for automatically washing the filter integrated into a single body in a filtering device. As multiple filtering units are used, ballast water treatment capacity is increased accordingly. Therefore, such a multi-cage type ballast water filtering device is mainly used in a large ship which must treat a large capacity of ballast water.
FIG. 1 is a side view of a conventional multi-cage type ballast water filtering device. FIG. 2 is a plan view of the conventional multi-cage type ballast water filtering device.
The conventional multi-cage type ballast water filtering device has a structure capable of automatically backwashing filters to remove foreign substances, etc. from the filters and discharging them out of the device. However, the conventional filtering device is problematic in that foreign substances, etc. are deposited on a backwash line during a backwashing process and back pressure attributable to the deposited foreign substances, etc. is increased, whereby the backwashing process may not be smoothly conducted. Also, there was no effort to fundamentally overcome such problems.
Furthermore, as shown in FIGS. 1 and 2, in the conventional filtering device, not only drive units a which operate backwash devices for backwashing filtering units but also discharge units b and discharge pipes c for discharging foreign substances out of the filtering device after the backwashing process all are installed on an upper part of the filtering device. As such, because many elements are gathered in one place, work for installation, replacement or disassembly of the elements is not easy. Particularly, under special environment conditions, that is, in a ship, space provided to install such a filtering device is very small (generally, to ensure sufficient space for original purposes of the ship, it is designed such that space such as a machinery room is relatively small). Therefore, given the fact that space defined above the upper part of the filtering device is also small, the conventional structure in which installation positions of many elements are focused on the upper part of the filtering device makes the maintenance of the filtering device more difficult.
In addition, the more the installation positions of the elements of the filtering device are focused on the upper part of the filter device, the more the space required above the upper part of the filtering device to install the elements or allow disassembly work must also be increased. Thus, an inefficient spatial structure is caused in that a separate upper space for the filtering device must be secured under special environment conditions, that is, in the ship.
Furthermore, in the conventional filtering device, if backwash water and foreign substances that have moved along the backwash line settle in a pipe of the backwash line when the force with which backwash water and foreign substances flow along the backwash line becomes insufficient, or if backwash water is concentrated in the backwash line, the degree of opening of the pipe of the backwash line is reduced. In this case, pressure in the backwash line, that is, back pressure, increases, thus reducing the backwashing efficiency and the filtering efficiency of the filtering device. However, the conventional filtering device has no technical structure to solve these problems.
Moreover, the conventional filtering device does not sufficiently have a means for fundamentally preventing the elements used in the backwash devices from colliding with each other or with deposited foreign substances and being damaged during a process of backwashing the filtering units. In addition, the conventional filtering device does not have any technical structure to solve problems derived from ballast water that remains in the filtering device while the operation thereof is interrupted, or problems pertaining to a seal of the filter for preventing ballast water that has not been filtered from leaking out of the filter.